The demands for better quality of signals in wireless communication and higher transmission speed/throughput fuel the rapid development of multi-antenna array technology that is applicable to communication devices, such as Multi-Input Multi-Output (MIMO) antenna system or beam-forming antenna array system technology. MIMO antenna system has the potential to increase spectrum efficiency and significantly increase channel capacity and data transmission speed. It also has the potential to enhance the reliability of receiving signals at the terminal communication devices. It has become one of the promising technology candidates used in upcoming fifth generation (5G) mobile communication system. For example, under an 8×8 MIMO system, the spectrum efficiency may reach about 37 bps/Hz (20 dB signal-to-noise ratio condition), which is about 4 times that of a 2×2 MIMO system.
However, it remains a challenge to realize a multi-antenna array system in a single space-limited handheld communication device while achieving good radiation characteristic and antenna efficiency for each individual antenna. This would be an important issue need to be solved in the near future. When a plurality of antennas operating in the same frequency band are co-designed and integrated in a communication device with limited space, the envelope correlation coefficient (ECC) between the multiple antennas would greatly increase, resulting in attenuation of the antenna radiation performance and a reduction in the speed/throughput of data transmission, making integration of multi-antenna design a challenging task.
Some previous technology documents have proposed a design scheme that increases energy isolation between multiple antennas by providing a protruding or recessed structure on a ground plane between the multiple antennas as an energy isolator. However, such a design may lead to excitation of additional coupling currents, causing an increase in the correlation coefficients between the multiple antennas, and possibly an increase in the overall size of the multi-antenna array. This is not desirable for commercial terminal communication devices, which require high efficiency and downsized multi-antenna array designs.
Therefore, there is a need for a design that solve the above-mentioned problems in order to meet the demand for high data transmission speed/throughput in future multi-antenna communication devices.